A variety of stents are known for use within arteries of a patient for treating stenoses, strictures, aneurysms, and the like. For example, a stent may be implanted within a partially occluded region of an artery to retain stenotic material beneath the stent and/or to open the lumen of the artery to improve blood flow therethrough.
Stents generally have a substantially cylindrical shape and are expandable between a contracted condition for facilitating delivery and an enlarged condition for engaging the vessel wall after deployment within the artery. Stents may be self-expanding, i.e., they may be biased to the enlarged condition but restrained in the contracted condition during delivery, for example within a sheath. Alternatively, stents may be substantially malleable or plastically deformable, i.e., the stent may be delivered in a contracted condition on a delivery catheter, and expanded by a balloon on the delivery catheter, until it plastically deforms into the enlarged condition.
Many stents include a plurality of segments or cells that are separated by one or more connectors extending between adjacent segments. For example, U.S. Pat. No. 5,104,404 discloses an expandable stent that includes a number of cylindrical segments, with single hinges connecting adjacent segments. Because of the rigidity of the individual segments of the stent, the hinges are intended to provide articulation between the adjacent segments.
When the stent is deployed within a curved portion of a vessel, the individual segments substantially resist bending to conform to the curvature of the vessel. The articulation provided by the hinges allows some conformity with the curvature of the vessel; however, the hinges may create gaps between the segments and/or may cause the segments to overlap one another. Material, such as stenotic material on the vessel wall, may extend through the gaps into the vessel lumen, possibly obstructing blood flow and/or breaking loose and traveling downstream where they may cause substantial damage to the patient being treated.
To reduce the likelihood of gaps occurring, some stents provide a number of connectors extending between adjacent segments. Increasing the number of connectors substantially, however, may increase the rigidity of the stent, which may cause problems during stent delivery. For example, when the stent is being delivered along a circuitous arterial path, the rigidity of the stent, particularly in its contracted condition, may impair advancement of the stent around tight bends in the artery.
Alternatively, some stents may include flexible connectors that are deformed when the segments are expanded to the enlarged condition. The resulting connectors may deform substantially to become part of the stent structure, i.e., they may deform substantially such that they lose their flexibility and are then unable to accommodate transverse bending forces.
In addition, some known stents have substantial gaps within the individual segments themselves or between the connectors, and so may not effectively "scaffold" the underlying vessel wall, i.e., may not support the vessel wall to maintain a desired open lumen cross-section and/or may expose material extending from the vessel wall into the bloodstream. Initially, stents in their contracted condition may have substantially few gaps within the individual segments, i.e., peripherally about the circumference of the segments. When the stents are expanded to their enlarged condition, however, substantial gaps may be created at one or points along the circumference either within the segments, due to the design of the segments or to uneven expansion of the individual segments, or between the connectors.
The risk of uneven radial expansion may be particularly problematic with respect to balloon-expandable stents due to the nature of the balloons generally used. Balloon-expandable stents are typically manually compressed onto an inelastic balloon provided on the delivery catheter. Because of its inelasticity, the balloon is typically rolled circumferentially or otherwise wrapped around the catheter before the stent is placed over it, thereby attempting to ensure that the balloon is not snagged or damaged during delivery or deployment of the stent.
Once the stent is delivered intraluminally to a desired region within a vessel, the balloon is inflated to expand the stent to its enlarged condition. As the balloon unwraps during inflation, it may subject the stent to radial forces that are not substantially even along the length and/or the circumference of the stent. More particularly, some regions of the balloon may expand more quickly than other regions that have not yet fully unwrapped, causing localized heightened radial forces which may cause uneven radial expansion of the stent. Because of the unevenly distributed forces, a portion of the circumference of the stent overlying an initially unwrapped region of the balloon may be expanded greater than an adjacent portion where the balloon has not yet fully unwrapped. This may substantially increase the risk of over-expanding portions of the stent, and thereby creating gaps in the over-expanded portions.
In an effort to provide a uniform enlarged condition, stents generally have a substantially uniform pattern extending about the circumference of the individual segments, and generally have segments of equal lengths. Because of the uneven radial forces which may be encountered during expansion, however, these stents may not expand substantially uniformly despite the intended result of their uniform designs. This problem may be further exacerbated because individual stents are generally intended to be expanded to a range of potential enlarged sizes, for example, between 3.0 mm and 5.5 mm. While at the upper end of the range, the radial forces may become more even and expand the stent more uniformly, the stent may be prone to uneven expansion at the lower end of the range, where localized heightened radial forces are more likely to occur.
Accordingly, it is believed that there is a need for stents which more effectively scaffold the vessel wall and/or which substantially evenly engage vessel walls, particularly within curved vessel regions, and for methods and systems using such stents.